The need for silent operation of ventilation systems is especially critical on board submarines, Naval surface vessels, HVAC systems, and other noise sensitive applications. Other locations, such as offices, ventilation systems require acoustic ceilings and other measures to reach acceptable noise criteria. In many cases, a major contributor to noise being generated is the ventilation fan. Axial ventilation fans are typically used in many applications because of their compact size and large flow capabilities.
Typically, in prior axial fan art, the inlet air flows directly into a rotating fan blade. In a compact duct fan, this can be preceded by an inlet cone, bellmouth or venturi section to accelerate the inlet air. The rotating fan blade imparts an axial and rotational (swirl) momentum component to the air. The fan discharge may include fixed turning vanes which removes the rotational or swirl component of the velocity but will increase noise at passing blade frequencies. A long discharge diffuser cone can also be provided to reduce velocity and improve static efficiency of the air distribution system; but also cause a total loss of pressure.
A uniform axial inlet air flow is required for maximum performance of prior art fans "without prerotation" (ASHRAE Handbook, 1983 Equipment Volume, Published by American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. Atlanta, GA. ISSN: 0737-0687, page 39). Quoting this reference further, "One of the most detrimental flow conditions is one which permits spin to develop in the air stream approaching the inlet to the fan irrespective of the type. A spin in the direction of impeller rotation reduces volume flow and pressure; a reverse spin may not have a large effect on volume flow, but the power requirement increases".
In the prior art, fan generated noise was generally a given fact, and various bulky and heavy silencers or duct designs were used to muffle and isolate the fan noise. Low noise prior art fan designs did not alter the basic uniform axial inlet air flow requirement, but altered the size, speed, mounting and structure to reduce noise. Discharge turning vanes were provided to eliminate outlet swirl which increased static performance somewhat but induced an added noise component at passing blade frequencies.
The silencers at low speed fans of the prior art required additional cost and space. The outlet turning vanes sometimes increased noise significantly. The effectiveness of these prior art measures at reducing noise is not always totally satisfactory. The silencers may be effective in normal noise frequencies, but only partially effective at others. Because of space limitations, silencers are not feasible in many installations. The addition of outlet turning vanes may be effective in reducing outlet swirl but can add additional fan noise at the blade passing frequency. Changes in discharge pressure or volume flow to the distribution system would generally require a long diffuser, resulting in more space and increased losses. In addition, if the selected fan/silencer combination did not meet the noise criteria, major changes were generally required.